Such arrangements are already known in the art, e.g., from DT-GMS (German Utility Patent) No. 7,336,864. Ball bearing balls are inexpensive, friction resistant and highly accurate. Their spherical shape insures that the probe does not measure incorrectly if the spherical sector produced from the ball bearing ball is not placed precisely polarly on the layer to be measured. With the probe already known in the art, the spherical sector has been welded to the topside of a plate of a threaded stem. During this welding, the spherical sector is heated and its permeability is changed in a nonreproducible manner. After welding, one may anneal both parts, so that a definite reproducible permeability can be achieved with this second operation. However, the spherical sector becomes mechanically soft and its wear resistance deteriorates.
The configuration of the known probe is such that, with the threaded stem screwed all the way into the magnetic core, the plate used for fastening the spherical sector makes contact with the magnetic core. Magnetically this means that the lines of force first pass through the spherical sector, then through the connecting layer between spherical sector and plate (which layer has magnetic leakage) and through the plate and only then to the front side of the core.
It is, therefore, an object of the present invention to create an arrangement where these disadvantages are avoided and which is less expensive to manufacture.